1. Field of the Invention
The present invention relates to a broadcast communication system and method for broadcasting broadcast media such as video information, audio information and the like, and to a communication terminal, a server, a relay apparatus, and a program, which are suitably used for the system and the method. Hereinafter, the word “broadcast” includes the meaning of “multicast”.
2. Description of the Related Art
In recent years, the digital satellite broadcast communication system has become widespread. In the digital satellite broadcast communication system, multi-channelization has developed by utilizing a communication band with a large capacity, and a large number of channels dedicated to sports, movie, music, news and the like have been prepared. In these dedicated channels, program contents in accordance with plans and contents of the respective fields have been broadcasted. Then, broadcast media enabling the reproduction (playback) of the program contents has been broadcasted by use of the digital satellite broadcast communication system.
As a broadcast communication system for broadcasting such broadcast media as video information, music information and the like, Japanese Patent Laid-Open Publication 2000-358062 discloses “Digital data transmitting/receiving system and method”, in which a variety of data such as broadcast media are transmitted by use of a data transmission format in conformity with the MPEG-2 system.
Moreover, as a broadcast communication system for repeatedly transmitting the variety of data such as broadcast media, Japanese Patent Laid-Open Publication 2001-53696 discloses a system for broadcasting a variety of data such as broadcast media by use of a “DSM-CC (Digital Storage Media Command and Control) data carousel system”, prescribed in the ISO/IEC13818-6. In other words, this Publication discloses “Section sending apparatus and section sending method in digital broadcast”, in which a variety of data such as broadcast media are repeatedly transmitted in a format which conforms to a section format prescribed in the ISO/IEC13818-1.
Meanwhile, as a broadcast communication system for the Internet, a broadcast communication system has been conceived for broadcasting broadcast media such as audio information, video information and the like, in which “information necessary to reproduce broadcast media (IP address information of a source and a destination, and the like)” is notified by use of a “SAP (Session Announcement Protocol) message” as broadcast notification information.
The SAP message is transmitted through a predetermined “IP address” and “port number”. Moreover, the SAP message is uniquely identified by “IP address information of a source (originating source) of the SAP message” and a “message ID uniquely specifying the SAP message transmitted by the source (msg id hash), which are included in the SAP message”.
In the broadcast communication system using the SAP message over the Internet, in general, a “multicast address (IP address information of a destination)” to which broadcast media is transmitted is designated to a “c field” in “SDP (Session Description Protocol) information” in the SAP message. A receiver can know the “multicast address (IP address information of the destination)” to which the broadcast media is transmitted by the “c field”, and thus can see and hear the broadcast media.
FIG. 45 illustrates an example of transmitting two broadcast media in a conventional broadcast communication system.
Specifically, FIG. 45 is an example of transmitting, on one channel, two “SAP messages (SAP1, SAP2)” and “RTP (Real-time Transport Protocol: RFC 1889) packets (RTP1-a and RTP1-b, RTP2-a and RTP2-b)” which correspond to the respective SAP messages (SAP1, SAP2).
FIG. 46 is a description example of the “SDP information” of the SAP message. FIG. 46 shows a “multicast address (IP address information of a destination)” of broadcast media in a portion of “c=IN IP 4 224.2.17.12/127”. Hence, a receiving terminal having received this SAP message can determine that the broadcast media is being broadcasted by the multicast address.
Moreover, in a packet-based communication system, a packet including audio and video information is transmitted as a RTP packet. A “RTP/UDP/IP header” as a header will be added to the RTP packet, the overhead thereof being extremely large.
As a method for reducing (compressing) the overhead of the RTP/UDP/IP header, “CRTP” prescribed in RFC2508 of IETF and “ROHC” prescribed in RFC3095 is known.
In any of the header compression methods, a “header compression refresh packet” for initializing a header compression state is first transmitted, and then a “header compression packet” in which the header is compressed is transmitted. In the “CRTP”, a “FULL_HEADER” corresponds to the header compression refresh packet, and in the “ROHC”, an “IR header” corresponds thereto.
In general, the digital satellite broadcast communication system having a communication band with a large capacity, and the broadcast communication system for the Internet are flexible, regarding the overhead in the repeated transmission of the broadcast media.
However, in a broadcast communication system including, as receiving terminals, mobile communication terminals that can only utilize a communication band with a relatively small capacity (low bit rate), there is an inherent problem in that the pressure for a transmission capacity of audio information, video information and the like is more significant by the above-described overhead.
Moreover, when the header compression method described above is simply applied to a packet-based broadcast communication system in order to reduce the above-described overhead, a receiving terminal that starts to see and hear the broadcast of the broadcast media from a halfway thereof has been unable to recover a header compression packet because the header compression state thereof is unclear even if the receiving terminal receives the header compression packet.
Moreover, the conventional broadcast communication system using the SAP message over the Internet has only one “physical channel” for use in transmitting the broadcast media, thus can see and hear the broadcast media by the designation of the IP address described in the “c field” of the “SDP information” in the SAP message. However, in the broadcast communication system using the mobile communication terminals, there has been a problem specifying to which wireless channel the broadcast media is broadcasted by only the designation of the IP address, because a plurality of broadcast media are conceived to be broadcasted by use of a plurality of wireless channels (physical channels).
Specifically, in the broadcast communication system using mobile communication terminals, there arises a subject concerning how the “physical channel” and a “logical address (IP address)”, which are for use in broadcasting the broadcast media, are to be associated with each other.
Furthermore, unlike a communication system for receiving and reproducing multimedia information (broadcast media) by an indication from the receiving terminal, in the reception and reproduction of the multimedia information (broadcast media) in the broadcast communication system, there is an inherent problem in that a broadcast media supplier (source) cannot know at which timing the receiver starts to receive the broadcast media, and sometimes cannot make the receiver reproduce (playback) the broadcast media reliably.